In the surgical treatment of cancer, and particularly in the treatment of breast cancer, the extend of intervention (e.g. mastectomy or ‘lumpectomy’) constitutes an important decision. The status of the lymph nodes—ascertained by biopsy during surgery—is a particularly important factor in the making of that decision. The sentinel lymph node (SN) is an important indicator of the spreading of the cancer cells from the primary tumour through the lymphatic system. To localize the SN with high accuracy intra-operatively, the radio guided technique is typically employed: a radioactive tracer based on Tc-99m (with a 141 keV gamma emission) is administered in the tumour, and appears—via the lymphatic drain—in the SN shortly thereafter. Locating the SN can be done intra-operatively with gamma probes in spectroscopy mode for identifying the gamma isotope.
Different types of gamma probes have been developed based on scintillator detectors and solid sate semiconductor detectors. These include probes based on a single CsI(Tl) or NaI(Tl) scintillator optically coupled to photomultipliers by fibre optics guides, and scintillators directly coupled to single photodiode silicon detectors (J. Chavanelle and M. Parmentier, “A CsI(Tl)-PIN photodiode gamma-ray probe,” Nucl. Instr. & Meth., 504 (2003) 321-324). This technology is used to localize small tumours during surgery or in conjunction with endoscopic examination.
Single scintillator gamma probes with a passive collimator (of Pb or Tungsten) are used to improve localization of deeply located radioactivity. This may be required owing to attenuation of scattered radiation or gamma photons from background radiation due to activity uptake in organs other than the tumour. Such gamma probes require high Z detector material (e.g. scintillator or CZT), and a detector module operating in a spectroscopy mode both so that energy windows suitable for the particular isotope can be selected and so that gamma photons from the isotope can better be discriminated from other radiation scattered in the patient's body. Such an approach is suitable for gamma sources with photon energies less then 200 keV. Localization of radioactive sources with high gamma energies—such as PET isotopes (511 keV)—using passive collimation is impractical owing to the thick collimator wall of such probes, which make the probes too bulky for many intraoperative applications.
Another approach, proposed by H. Watabe et al. (“Development of a miniature gamma-ray endoscopic probe for tumour localization in nuclear medicine,” IEEE Trans. on Nucl. Sci. 40 (1993) 88-94), attempts to improve spatial resolution of source localization in cases of gamma photons with energy greater than 200 keV. This approach employs dual BGO (B4Ge3O12) scintillators closely coupled to each other but without a collimator, each of the two scintillators being connected with a fibre optic light pipe to its own photomultiplier. The random coincidence technique is then used, on the basis that the probability of a random coincidence of events in the two scintillators is P1×P2, where P1 and P2 are the probabilities of gamma photon registration by the first and second detectors respectively. It is clear that, for localized activity, the number of random coincidence events decreases more rapidly with distance than the number of events for a single detector, and this effect can potentially be used to achieve better spatial resolution. The disadvantage of this approach—whether using a single or dual scintillator detectors—is that photopeak resolution deteriorates owing to the coupling of the fibre optic light pipe and the photomultiplier. This is due to loss of photons and hence deterioration in the spatial resolution of the probe.
The use of two concentric collimated scintillator detectors coupled by light pipes to two photomultiplier has been proposed by T. S. Hickernell et al. (“Dual-Detector probe for Surgical tumour staging”, J. Nucl. Med 29 (1988) 1101-1106), where the outer detector suppresses background gamma activity. However, this probe has limitations for large tumours determinations, in addition to the aforementioned disadvantages in the coupling of scintillators to photomultipliers with light pipes.
PET isotopes (i.e. mixed field beta and 511 keV gamma field) or pure beta isotopes (e.g. P-32) for labelling cancer cells improves localization of activity (and hence of the tumour) owing to their short range compared to gamma radiation. However, gamma background activity from other parts of the body remains a problem for the accurate detection of beta particles on a gamma background.
U.S. Pat. No. 5,008,546 proposes a beta probe with improved selectivity in a mixed gamma-beta field. This probe employs two closely coupled plastic scintillators, or concentric scintillators where the outer scintillator is protected from beta radiation by shielding (e.g. 1 mm thick stainless steel). The plastic scintillators are optically coupled by fibre optics bundles to respective photomultipliers. Photomultipliers employ high bias voltages (of the order of 1000 V) and are separated from their corresponding scintillators. Radiative counts produced by pure beta radiation in an unshielded plastic scintillator are obtained by weighted subtraction of the count from the second plastic scintillator.
Low Z scintillators have the advantage when used in detector modules for beta detection of having low efficiency for the detection of high energy gamma radiation. However, they do not allow such probes to be used as efficient gamma probes in spectroscopy mode.
R. R. Raylman and A. Hyder (“A dual surface barrier detector unit for beta-sensitive endoscopic probes”, IEEE Trans on Nucl. Sci., 51 (2004) 117-121) report the use of dual detectors for beta detection in a high energy gamma background of a PET (FDG, that is, F-18) isotope for an endoscopic probe based on two miniature 3 mm diameter Si surface barrier detectors mounted back to back. Gamma background can be subtracted from the front detector by weighted count output of the back detector. While the detector module is small, the disadvantages of such a probe arise from the complicated mounting of detector and read out electronics and its inutility as a gamma spectroscopy intraoperative probe for high energy gamma isotopes, such as Tc-99m.
The identification of the signature of radioactive isotopes with medium and low energy gamma photons can be difficult in the presence of high energy masking gamma isotopes, as the monitored events may be statistically poor compared with the Compton scattering background from higher energy masking isotopes. This problem has been addressed with the nuclear spectroscopy technique of anti-Compton spectrometry, in which events from a primary detector are rejected if associated with interactions via the Compton scattering process. Such events produce no usable signal for spectroscopy purposes.
The rejection is achieved by surrounding the primary detector with an active Compton shield, which detects the Compton scattered photons originating from within the primary detector thereby producing a veto signal for the primary detector data acquisition system. This results in the suppression of the Compton continuum in the primary detector spectrum and hence to a greater probability of detection of lower energy gamma photons otherwise statistically lost in the Compton background. It also allows the use of smaller primary spectroscopy grade detectors while maintaining a good photopeak-to-compton (P/C) ratio for a wide range of photon energies.
R. Aryaeinejad et al. (“High resolution Compton-suppressed CZT and LaCl3 detectors for fission identification,” IEEE Trans on Nucl. Sci., 52(5) (2005) 1728-1732) propose an anti-Compton spectrometer for the detection of fissile materials using a 10×10×5 mm3 CZT primary detector and a NaI(Tl) or BGO veto detector. The primary and veto detectors thus operate on different principles, the former being semiconductor based, the latter scintillator based. This is inconvenient, requiring both photomultipliers and solid state detector readout electronics as well as different high voltage biasing circuitry. It also leads to a large volume of inactive material, a so-called “dead layer,” between the primary and veto detectors that reduces Compton suppression efficiency. One proposal to alleviate this problem involves connecting the CZT primary detector to a preamplifier located externally to the veto detector, but this degrades the main advantage of CZT, namely, high energy resolution (reported to be 4.6% for 661.7 keV).
U.S. Pat. No. 6,710,349 discloses a detector with a position sensitive radiation detector having a radiation sensitive area and a radiation insensitive area, a first scintillator adjacent to the radiation sensitive area and a second scintillator adjacent to the second radiation insensitive area. The second scintillator is optically coupled to the first scintillator, and the decay times of the first and second scintillators are different.